Semiconductor colloidal nanoparticles such as quantum dots and quantum rods are promising color-tunable light emitters due to the fact that their effective bandgaps can be synthetically manipulated by controlling the particle size and shape. Since a large fraction of the atoms in such a nanoparticle are placed on the particle surface, optical properties of the nanoparticles depend strongly on the surface environment thereof.
Accordingly, in order to prepare highly luminescent nanoparticles, there has been widely employed a passivation technique to put an inorganic overcoat on the surface of a nanoparticle core, the overcoat being a chemically grown shell. Nanoparticles with an inorganic overcoat are often called “core/shell structure nanoparticles”. CdSe/ZnS core/shell structure nanoparticles are particularly preferred in many applications due to their high photoluminescence quantum yield and high stability.
Nanoparticles having a core/shell structure are generally prepared by slowly adding a solution containing a shell material precursor into a solution prepared by dissolving a core nanoparticle in a reaction solvent, which is called a “dropwise method”. The reaction solvent used above comprises one or more amphiphilic molecules having long alkyl chains and polar head groups. As these amphiphiles act not only as a reaction solvent but also as capping molecules which stabilize the nanoparticle surface, the aforementioned reaction solvent is also referred to as a coordinating solvent.
However, such a conventional dropwise method has problems in that the stability of the core in such a reaction solvent is easily affected by small changes in reaction conditions such as the temperature, solvent composition and concentration. For example, when the core solution is heated to a temperature suitable for shell growth before or during the addition of the shell precursor solution, the core undergoes dissolution and/or ripening, which degrades its original properties such as the emission quantum yield and emission band width. Herein, the term ‘ripening’ means that nanoparticles in the solution gradually become larger, and at the same time, the size distribution thereof becomes substantially wide. Further, in order to suppress undesirable self-nucleation of the shell precursors which leads to a non-uniform overcoat, the addition rate of the shell precursor solution must be kept a very low level.
In practice, however, the core dissolution and ripening processes, and the self-nucleation of the shell precursors always occur to some extent, competing with the desired core/shell formation. In other words, the conventional dropwise method yields core/shell structure nanoparticles whose properties vary depending on the synthesis batch.